The ABC's of Copper Bactericides for Vegetables

Activity - How does copper work against pathogens?

Bacteria - At what stage can we have an effect?

Copper formulations - Are some better than others?

Here are a few points to help you design the best copper bactericide program for your operation.

Activity

As you probably already know, a higher % copper in the product does not necessarily translate to more activity against pathogens. For example, Kocide DF had 40% metallic copper, but Kocide 2000 has only 35% metallic copper. However, the company's literature states that Kocide 2000 has "over 2.5 times the bio-available copper of Kocide DF".

How can this be? Research has shown that efficacy is not related to the amount of metallic copper in the product, but is usually related to the "free cupric ions" (Cu2+) in solution or on the leaves.

So, when you apply a copper bactericide, you are depositing insoluble copper salts on the plant. On the foliage, small amounts of Cu2+ are released - the amount depends on formulation, weather, foliage characteristics, and other factors. It is these Cu2+ ions that are active against bacterial pathogens.

Research has also shown that tank-mixes of copper bactericides with mancozeb fungicides increase the quantity of Cu2+ in solution and have been shown to increase control of bacterial disease compared to using the copper product alone.

Bacteria

What effect do the copper bactericides have on bacterial disease? These products do not eradicate bacteria. They can reduce the bacterial population on the plant, but do not seem to reduce the growth rate of the bacterial population. This is why it is so important to apply them early in the growing season, before bacterial populations have a chance to explode. We don't have any effective options once an outbreak has occurred.

Copper Formulations

As mentioned above, activity of the copper bactericides against bacterial disease depends on how many free cupric ions (Cu2+) we can get into solution to attack the bacterial cells.

Some products perform better than others in field studies, but results vary somewhat between studies. Generally, the copper formulations registered on tomato and pepper bacterial disease in Canada all perform adequately and have data to back them up. Although there are more products registered in the US, some have not shown much efficacy in research trials and so it's probably a good thing that they would not make the grade for registration in Canada. There are actually products registered against bacterial diseases in the US that have less than 5% copper! Not surprisingly, they have not performed well compared to the industry standards with their 25% to 50% and higher levels of fixed copper.